Structural biology of chemokine receptors

Structural dynamics of chemokine receptors

Membrane proteins such as G protein-coupled receptors (GPCRs) are involved in awide range of physiological and pathological cellular processes. Binding of extracellular signals to GPCRs, including hormones, neurotransmitters, peptides and proteins, can activate intracellular signaling cascades via G protein interaction. Chemokine receptors are key GPCRs implicated in cancers, immune responses, cell migration and inflammation. Specifically, the CCR5 and CXCR4 chemokine receptors serve as important therapeutic targets against Human Immunodeficiency virus (HIV) entry into human cells. Maraviroc and Vicriviroc, two clinically used HIV entry inhibitors, are antagonists of the CCR5 receptor. These drugs block HIV entry, but ultimately resistance develops, due to emergence of viruses that can utilize the CXCR4 co-receptor. Unfortunately, development of chemokine receptor antagonists as selective drugs of HIV infection has been greatly hindered as their target orthosteric site is conserved among different receptor subtypes. Accordingly, it is important to understand the structural dynamics of these receptors to develop more effective therapeutics. In this chapter, we describe the latest advances in studies of these two key chemokine receptors with respect to their structures, dynamics and function.

Kinetic and thermodynamic studies reveal chemokine homologues CC11 and CC24 with an almost identical tertiary structure have different folding pathways

BACKGROUND

Proteins with low sequence identity but almost identical tertiary structure and function have been valuable to uncover the relationship between sequence, tertiary structure, folding mechanism and functions. Two homologous chemokines, CCL11 and CCL24, with low sequence identity but similar tertiary structure and function, provide an excellent model system for respective studies.

RESULTS

The kinetics and thermodynamics of the two homologous chemokines were systematically characterized. Despite their similar tertiary structures, CCL11 and CCL24 show different thermodynamic stability in guanidine hydrochloride titration, with D_50% = 2.20 M and 4.96 M, respectively. The kinetics curves clearly show two phases in the folding/unfolding processes of both CCL11 and CCL24, which suggests the existence of an intermediate state in their folding/unfolding processes. The folding pathway of both CCL11 and CCL24 could be well described using a folding model with an on-pathway folding intermediate. However, the folding kinetics and stability of the intermediate state of CCL11 and CCL24 are obviously different.

CONCLUSION

Our results suggest homologous proteins with low sequence identity can display almost identical tertiary structure, but very different folding mechanisms, which applies to homologues in the chemokine protein family, extending the general applicability of the above observation.

Macrophage Inflammatory Protein-3 Alpha (MIP-3α)/CCL20 in HIV-1-Infected Individuals

OBJECTIVE

Uncontrolled HIV infection progresses to the depletion of systemic and mucosal CD4 and AIDS. Early HIV infection may be associated with increases in the concentration of MIP-3α in the blood and gut fluids. MIP-3α/CCL20 is the only chemokine known to interact with CCR6 receptors which are expressed on immature dendritic cells and both effector and memory CD8⁺ and CD4⁺ T cells. The role and prognostic value of blood levels of MIP-3α in HIV-infected individuals has yet to be described.

METHODS

We determined the serum levels of MIP-3α, and IFN-γ, in 167 HIV-1-infected and 27 HIV-1-uninfected men participating in the Multicenter AIDS Cohort Study (MACS). The blood biomarkers were measured using enzyme-linked immunosorbent assays (ELISA) and the cell phenotypes using flow cytometry.

RESULTS

Median serum levels of MIP-3α in HIV-1-infected and uninfected men was significantly different (p<0.0001) and were 21.3 pg/mL and 6.4 pg/mL respectively. The HIV-1-infected men with CD4⁺ T cell count <200 cells/μL showed the highest median serum MIP-3α (23.1 pg/mL). Serum levels of MIP-3α in HIV-1 infected (n=167) were negatively correlated with absolute number of CD4⁺ T cell (p=0.01) and were positively correlated with CD38 molecules on CD8⁺ T cells (p=0.0002) and with serum levels of IFN-γ (0.006).

CONCLUSION

Serum levels of MIP-3α concomitantly increase with plasma levels of IFN-γ, CD38 expression on CD8⁺ T cells, and decreased of absolute CD4⁺ T cells in HIV-1-infected men. A higher blood level of MIP-3α may be representation of locally high level of MIP-3α and more recruitment of immature dendritic cell at site of infection. Involvement of CCR6/CCL20 axis and epithelial cells at the recto-colonel level may enhance sexual transmission of HIV-1 in MSM and may be useful as a prognostic marker in HIV-1-infection and AIDS.

Comparative analysis of gene transcripts for cell signaling receptors in bone marrow-derived hematopoietic stem/progenitor cell and mesenchymal stromal cell populations

Introduction

Knowing the repertoire of cell signaling receptors would provide pivotal insight into the developmental and regenerative capabilities of bone marrow cell (BMC)-derived hematopoietic stem/progenitor cells (HSPCs) and bone marrow mesenchymal stromal cells (BMMSCs).

Methods

Murine HSPCs were enriched from fluorescence-activated cell sorting (FACS)-sorted Lin^–c-Kit⁺Sca-1⁺ BMCs isolated from the tibia and femoral marrow compartments. Purified BMMSCs (CD73⁺, CD90⁺, CD105⁺, and CD45^–, CD34^–, CD31^–, c-Kit^–) with extensive self-renewal potential and multilineage differentiation capacity (into different mesodermal cell lineages including osteocytes, chrondrocytes, adipocytes) were derived from adherent BMC cultures after CD45⁺ cell depletion. Adherent colony-forming cells were passaged two to three times and FACS analysis was used to assess cell purity and validate cell-specific surface marker phenotype prior to experimentation. Gene transcripts for a number of cell signaling molecules were assessed using a custom quantitative real-time RT-PCR low-density microarray (94 genes; TaqMan^® technology).

Results

We identified 16 mRNA transcripts that were specifically expressed in BMC-derived HSPC (including Ptprc, c-Kit, Csf3r, Csf2rb2, Ccr4, Cxcr3 and Tie-1), and 14 transcripts specifically expressed in BMMSCs (including Pdgfra, Ddr2, Ngfr, Mst1r, Fgfr2, Epha3, and Ephb3). We also identified 27 transcripts that were specifically upregulated (≥2-fold expression) in BMMSCs relative to HSPCs (Axl, Bmpr1a, Met, Pdgfrb, Fgfr1, Mertk, Cmkor1, Egfr, Epha7, and Ephb4), and 19 transcripts that were specifically upregulated in HSPCs relative to BMMSCs (Ccr1, Csf1r, Csf2ra, Epor, IL6ra, and IL7r). Eleven transcripts were equally expressed (<2-fold upregulation) in HSPCs and BMMSCs (Flt1, Insr, Kdr, Jak1, Agtrl1, Ccr3, Ednrb, Il3ra, Hoxb4, Tnfrsf1a, and Abcb1b), whilst another seven transcripts (Epha6, Epha8, Musk, Ntrk2, Ros1, Srms, and Tnk1) were not expressed in either cell population.

Conclusions

We demonstrate that besides their unique immunophenotype and functional differences, BMC-derived HSPCs and BMMSCs have different molecular receptor signaling transcript profiles linked to cell survival, growth, cell differentiation status, growth factor/cytokine production and genes involved in cell migration/trafficking/adhesion that may be critical to maintain their pluripotency, plasticity, and stem cell function.

Loss of CD26 protease activity in recipient mice during hematopoietic stem cell transplantation results in improved transplant efficiency

BACKGROUND

A firm understanding of the biology of hematopoietic stem and progenitor cell (HSC/HPC) trafficking is critical to improve transplant efficiency and immune reconstitution during hematopoietic stem cell transplantation (HSCT). Our earlier findings suggested that suppression of CD26 (dipeptidyl peptidase IV) proteolytic activity in the donor cell population can be utilized as a method for increasing transplant efficiency. However, factors in the recipient should not be overlooked, given the potential for the bone marrow (BM) microenvironment to regulate HSCT.

STUDY DESIGN AND METHODS

We first evaluated CD26 expression and then investigated the effects of the CD26 inhibitor diprotin A and the absence of CD26 (CD26-/-) in recipient mice on HSC/HPC homing and engraftment using an in vivo congenic mouse model of HSCT.

RESULTS

A significant increase in donor cell engraftment into the peripheral blood (PB), and to a lesser extent homing into the BM, was observed in CD26-/- mice or CD26 inhibitor-treated mice. Increased PB engraftment of CD26-/- mice was significant at 3 and 6 months, but not 1 month, after transplant. It was noted that the increased homing was statistically greater with donor cell manipulation (CD26-/- donor cells) than with recipient manipulation (CD26-/- recipient mice). Conversely, donor and recipient manipulation both worked well to increase PB engraftment at 6 months.

CONCLUSION

These results provide preclinical evidence of CD26, in the HSCT recipient, as a major regulator of HSC/HPC engraftment with minor effects on HSC/HPC homing and suggest the potential use of CD26 inhibitors in HSCT patients to improve transplant efficiency.

In vivo expansion of the megakaryocyte progenitor cell population in adult CD26-deficient mice

OBJECTIVE

Megakaryopoiesis involves commitment of hematopoietic stem cells (HSC) toward the myeloid lineage in combination with the proliferation, maturation, and terminal differentiation of progenitors into megakaryocytes. The exact mechanism of megakaryocyte development from HSC is unknown, but growth factors such as thrombopoietin have been identified as critical. Additionally, it has been suggested that the chemokine CXCL12/stromal-cell derived factor-1α has a role in regulating megakaryopoiesis and thrombopoiesis. We recently reported the importance of the extracellular protease CD26 (dipeptidylpeptidase IV) in regulating HSC responses to CXCL12, as well as modulating HSC trafficking into and out of the bone marrow. However, the importance of CD26 for megakaryopoiesis has not been reported. We therefore compared megakaryocyte development between CD26-deficient (CD26(-/-)) mice and C57BL/6 control mice.

MATERIALS AND METHODS

Adult CD26(-/-) mice and C57BL/6 control mice were evaluated using blood differentials, histological analysis, flow cytometric analysis, and progenitor colony assays.

RESULTS

Bone marrow from CD26(-/-) mice has a significantly expanded megakaryocyte and megakaryocyte progenitor population compared to control C57BL/6 mice bone marrow.

CONCLUSIONS

Our results indicate that endogenous CD26 normally suppresses megakaryopoiesis and that loss of CD26 activity results in expansion of the megakaryocyte progenitor population in vivo. This suggests the potential use of CD26 inhibitors to improve megakaryocyte progenitor function and/or reconstitution of the megakaryocyte cell population.

Altered expression of the receptor-ligand pair CXCR5/CXCL13 in B cells during chronic HIV-1 infection

HIV-1 infection is associated with B-cell abnormalities, such as hypergammaglobulinemia, poor immunization responses, and loss of serologic memory. To determine whether altered expression of chemokine receptors and their ligands may play a role in B-cell dysfunctions during HIV-1 infection, the expression of CXC chemokine receptor 4 (CXCR4), CXCR5, and CC chemokine receptor 7 (CCR7) and their respective ligands on CD19(+) B cells were examined in HIV-1-infected patients and controls. We report a decreased CXCR5 expression on B cells from patients (P < .05), a phenomenon associated with a low CD4 T-cell count (< 350 cells/microL). Interestingly, an increased expression of CXC chemokine ligand 13 (CXCL13), the ligand for CXCR5, was found in peripheral B cells from HIV-1-infected patients. Moreover, on B-cell activation in vitro, CXCL13 was secreted in culture. CXCL13(+) B cells were also found in the lymph nodes of HIV-1-infected patients, but not in control tissue. B-cell migration toward CXCL13, CXCL12, and CC chemokine ligand 21 (CCL21), ligands for CXCR5, CXCR4, and CCR7 was also evaluated. In patients with a low CD4 T-cell count, migration toward all ligands was increased. Our findings indicate that altered expression of the chemokine receptor-ligand pair, CXCR5/CXCL13, may participate in the establishment of B-cell dysfunctions during HIV-1 infection.

Expanded cells in monoclonal TCR-alphabeta+/CD4+/NKa+/CD8-/+dim T-LGL lymphocytosis recognize hCMV antigens

Recent studies suggest the potential involvement of common antigenic stimuli on the ontogeny of monoclonal T-cell receptor (TCR)-alphabeta(+)/CD4(+)/NKa(+)/CD8(-/+dim) T-large granular lymphocyte (LGL) lymphocytosis. Because healthy persons show (oligo)clonal expansions of human cytomegalovirus (hCMV)-specific TCRVbeta(+)/CD4(+)/cytotoxic/memory T cells, we investigate the potential involvement of hCMV in the origin and/or expansion of monoclonal CD4(+) T-LGL. Peripheral blood samples from patients with monoclonal TCR-alphabeta(+)/CD4(+) T-LGL lymphocytosis and other T-chronic lymphoproliferative disorders were evaluated for the specific functional response against hCMV and hEBV whole lysates as well as the "MQLIPDDYSNTHSTRYVTVK" hCMV peptide, which is specifically loaded in HLA-DRB1*0701 molecules. A detailed characterization of those genes that underwent changes in T-LGL cells responding to hCMV was performed by microarray gene expression profile analysis. Patients with TCR-alphabeta(+)/CD4(+) T-LGL displayed a strong and characteristic hCMV-specific functional response, reproduced by the hCMV peptide in a subset of HLA-DRB1*0701(+) patients bearing TCRVbeta13.1(+) clonal T cells. Gene expression profile showed that the hCMV-induced response affects genes involved in inflammatory and immune responses, cell cycle progression, resistance to apoptosis, and genetic instability. This is the first study providing evidence for the involvement of hCMV in the ontogeny of CD4(+) T-LGL, emerging as a model disorder to determine the potential implications of quite a focused CD4(+)/cytotoxic immune response.

Acidic pH stimulates the production of the angiogenic CXC chemokine, CXCL8 (interleukin-8), in human adult mesenchymal stem cells via the extracellular signal-regulated kinase, p38 mitogen-activated protein kinase, and NF-kappaB pathways

Blood vessel injury results in limited oxygen tension and diffusion leading to hypoxia, increased anaerobic metabolism, and elevated production of acidic metabolites that cannot be easily removed due to the reduced blood flow. Therefore, an acidic extracellular pH occurs in the local microenvironment of disrupted bone. The potential role of acidic pH and glu-leu-arg (ELR(+)) CXC chemokines in early events in bone repair was studied in human mesenchymal stem cells (hMSCs) treated with medium of decreasing pH (7.4, 7.0, 6.7, and 6.4). The cells showed a reciprocal increase in CXCL8 (interleukin-8, IL-8) mRNA levels as extracellular pH decreased. At pH 6.4, CXCL8 mRNA was induced >60x in comparison to levels at pH 7.4. hMSCs treated with osteogenic medium (OGM) also showed an increase in CXCL8 mRNA with decreasing pH; although, at a lower level than that seen in cells grown in non-OGM. CXCL8 protein was secreted into the medium at all pHs with maximal induction at pH 6.7. Inhibition of the G-protein-coupled receptor alpha, G(alphai), suppressed CXCL8 levels in response to acidic pH; whereas phospholipase C inhibition had no effect on CXCL8. The use of specific mitogen-activated protein kinase (MAPK) signal transduction inhibitors indicated that the pH-dependent increase in CXCL8 mRNA is due to activation of ERK and p38 pathways. The JNK pathway was not involved. NF-kappaB inhibition resulted in a decrease in CXCL8 levels in hMSCs grown in non-OGM. However, OGM-differentiated hMSCs showed an increase in CXCL8 levels when treated with the NF-kappaB inhibitor PDTC, a pyrrolidine derivative of dithiocarbamate.

CD26 inhibition on CD34+ or lineage- human umbilical cord blood donor hematopoietic stem cells/hematopoietic progenitor cells improves long-term engraftment into NOD/SCID/Beta2null immunodeficient mice

Given the tremendous need for and potential of umbilical cord blood (CB) to be utilized as a donor source for hematopoietic stem cell (HSC) transplantation in adults, there is a strong push to overcome the constraints created by the limited volumes and subsequent limited HSC and hematopoietic progenitor cell (HPC) numbers available for HSC transplantation from a single collection. We have previously described the use of CD26 inhibitor treatment of donor cells as a method to increase the transplant efficiency of mouse HSCs and HPCs into a mouse recipient. To study the use of CD26 inhibitors as a method of improving the transplantation of human CB HSCs and HPCs, we utilized the nonobese diabetic/severe combined immunodeficient/beta 2 microglobulin null (NOD/SCID/B2m(null)) immunodeficient mouse model of HSC transplantation. We report here significant improvements in the engraftment of long-term repopulating cells following the treatment of either CD34(+) or lineage negative (lin()) donor CB with the CD26 inhibitor, Diprotin A, prior to transplant. These results establish a basis on which to propose the use of CD26 inhibitor treatment of donor CB units prior to transplantation for the purpose of improving transplant efficiency and subsequently patient outcome.

Modeling the structural basis of human CCR5 chemokine receptor function: from homology model building and molecular dynamics validation to agonist and antagonist docking

This article describes the construction and validation of a three-dimensional model of the human CCR5 receptor using a homology-based approach starting from the X-ray structure of the bovine rhodopsin receptor. The reliability of the model is assessed through molecular dynamics and docking simulations using both natural agonists and a synthetic antagonist. Some important structural and functional features of the receptor cavity and the extracellular loops are identified, in agreement with data available from site-directed mutagenesis. The results of this study help to explain the structural basis for the recognition, activation, and inhibition processes of CCR5 and may provide fresh insights for the design of HIV-1 entry blockers.

NMR structure of CXCR3 binding chemokine CXCL11 (ITAC)

CXCL11 (ITAC) is one of three chemokines known to bind the receptor CXCR3, the two others being CXCL9 (Mig) and CXCL10 (IP-10). CXCL11 differs from the other CXCR3 ligands in both the strength and the particularities of its receptor interactions: It has a higher affinity, is a stronger agonist, and behaves differently when critical N-terminal residues are deleted. The structure of CXCL11 was determined using solution NMR to allow comparison with that of CXCL10 and help elucidate the source of the differences. CXCL11 takes on the canonical chemokine fold but exhibits greater conformational flexibility than has been observed for related chemokines under the same sample conditions. Unlike related chemokines such as IP-10 and IL-8, ITAC does not appear to form dimers at millimolar concentrations. The origin for this behavior can be found in the solution structure, which indicates a beta-bulge in beta-strand 1 that distorts the dimerization interface used by other CXC chemokines.

Cell surface receptors, virus entry and tropism of primate lentiviruses

Human immunodeficiency virus (HIV) exploits cell surface receptors to attach to and gain entry into cells. The HIV envelope spike glycoprotein on the surface of virus particles binds both CD4 and a seven-transmembrane coreceptor. These interactions trigger conformational changes in the envelope spike that induce fusion of viral and cellular membranes and entry of the viral core into the cell cytoplasm. Other cell surface receptors also interact with gp120 and aid attachment of virus particles. This review describes these receptors, their roles in HIV entry and their influence on cell tropism.

Chemokine regulation of normal and pathologic immune responses

Chemokines are small basic proteins that are the major mediators of all leukocyte migration. There are at least 46 distinct chemokines, and 19 chemokine receptors, making it easily the largest cytokine family. Chemokines can be both beneficial and harmful, by either stimulating an appropriate immune response to microbial invasion, or by mediating pathologic tissue destruction in many types of human disease. Chemokines have been implicated in the tissue destruction seen in autoimmune diseases, atherosclerosis, allograft rejection, and neoplasia. Chemokines also play essential roles in normal lymphocyte trafficking to primary and secondary lymphoid organs for antigen presentation and lymphocyte maturation. Chemokines also regulate hematopoietic stem and progenitor cell homing and proliferation. Therefore, it is likely that chemokines will become important targets for pharmacologic intervention in a wide variety of human diseases in the future.

Transgenic overexpression of the CC chemokine CCL21 disrupts T-cell migration

Chemokines are a large family of cytokines that direct normal leukocyte migration. They also have been implicated in leukocyte development and in the pathogenesis of many diseases. The CC chemokine CCL21, also known as Exodus-2, SLC, 6Ckine, and TCA4 induces both the adhesion and migration of human T cells. CCL21 is hypothesized to regulate the trafficking of T cells through secondary lymphoid tissues. To test this hypothesis, a transgenic mouse model was generated that placed the expression of mouse CCL21 (mCCL21) under the control of the T cell-specific lck promoter to abrogate the concentration gradient to which T cells normally respond. Overexpression of mCCL21 in T cells resulted in defects in CCL21- and CCL19-induced T-cell chemotaxis, node T-cell subpopulations, and lymph node architecture. The regulation of T-cell trafficking in secondary lymphoid tissues by CCL21 is therefore a tightly regulated system that can be altered by changes in the level of environmental CCL21 protein.